Semiconductor SLs are multi-layered structures with unique electronic properties. These structures comprise a periodic stack of alternating thin (e.g., nanometer thick) layers of two different semiconductor materials having different bandgaps (i.e., lower bandgap quantum well (QW) layers interleaved with wider bandgap barrier layers). As described by G. Scamarcio et al., Science, Vol. 276, pp. 773-776 (May 1997), the period of this structure (.about.5 nm) is typically much larger than the lattice constant of the bulk crystalline constituents (.about.0.5 nm). This superimposed potential splits the conduction and valence bands into a series of much narrower bands (typically tens to a few hundred millielectron volts wide in the strong tunnel-coupling regime) called minibands, which are separated by energy gaps (minigaps) along the direction perpendicular to the layers. To form a miniband requires that the wavefunctions of the states in each of the QWs are delocalized; i.e., the wavefunctions extend over many QWs, thus indicating that the QWs are strongly coupled to one another; they are not localized in which case the QWs would be effectively decoupled from one another.
An interesting feature of SLs, their long inter-miniband relaxation time compared to the intra-miniband one, has been used to develop an SL QC laser with intrinsic population inversion and very large current carrying capabilities and optical power outputs. In these QC lasers the active region repeat units (radiative transition (RT) regions plus interleaved injection/relaxation (I/R) regions) are formed by SLs. See, for example, Scamarcio et al., supra, and A. Tredicucci et al., Appl. Phys. Lett., Vol. 72, No. 19, pp. 2388-2390 (May 1998), both of which are incorporated herein by reference. In both cases laser action in each RT region was achieved between minibands through unipolar (electron) injection by miniband transport through each I/R region. The vertical laser transition, between energy states at the bottom of an upper conduction miniband and empty states near the top of a lower conduction miniband, took place at a photon energy well below the energy band gap of the barrier and QW materials. The center wavelength of these QC SL lasers is determined by the minigap and can be selected over a large region of the IR spectrum by changing the barrier and QW thicknesses.
In order for QC SL lasers to properly function a flatband condition of the minibands must exist; i.e., two conditions should be met: (1) macroscopic alignment of the RT and I/R regions with one another, and (2) microscopic alignment of the upper and lower laser energy levels across the RTs. However, in the presence of the applied field (e.g., the external bias applied transverse to the layers to induce lasing) the quantum states, from QW layer to QW layer, shift to higher and higher energies in the direction of the field. In the Scamarcio et al. QC SL laser this problem was addressed by heavily doping the entirety of all of the RT regions so that the dopant ions and corresponding extrinsic electrons produced a screening field which compensated the applied field (i.e., prevented significant field penetration into the active region). On the other hand, in the Tredicucci et al. QC SL laser, only the ends of the I/R regions close to the RT regions were doped. Here, the dopant ions and their extrinsic electrons acted like opposing plates of a capacitor to screen the applied field. In this fashion, the SL regions were almost field free, with the upper miniband of each repeat unit aligned with the lower miniband of the previous, adjacent unit (in the direction of electron flow). However, the relatively high concentration of extrinsic electrons (sheet density per period of 7-8.times.10.sup.11 cm.sup.-2 in the Tredicucci et al. laser) resulted in high absolute values of threshold current as well as rapidly increasing threshold current with increasing temperature.
Thus, a need remains in the QC SL laser art for a design which achieves the desired flatband condition of the upper and lower minibands without requiring essentially field-free SLs; e.g., without the need to introduce dopants to compensate the applied field.